1. Field
Embodiments of the disclosure relate generally to the field of fastener technologies for metal fasteners employed in composite structures and more particularly to a fastener with a spherical head shaped such that it mates with a spherical washer containing a polymeric ring mechanically attached to its base to create a pressure contained system preventing hot particle ejection and edge sparking during lightning conditions.
2. Background
Structural fabrication using composites still requires fasteners for many assembly operations. Metallic fasteners are conductive creating electromagnetic effect (EME) design considerations for lightning strike and other EME issues. Fastener sparking modes must be designed for Lightning conditions include Hot Particle Ejection (HPE) and arcing between an outer surface of the fastener head and other structure or fastener sleeves.
When lightning strikes a Carbon Fiber Reinforced Polymer (CFRP) structure, a significant portion of the current may pass into nearby structures through fasteners. When the electrical energy passes between two surfaces, contact resistance heating may break down the materials and generate hot gas (or plasma) in the hole, or in the space in-between a fastener and an associated internally threaded fastener such as a nut or frangible collar. If the pressure generated is large enough, the gas may find a low pressure path, and escape from the fastener system into a fueled area. That path is sometimes in the surface under the protruding fastener head. These escaping gasses may be hot enough to erode the metal parts (fastener or structure) as they escape, creating particles or droplets known as HPE.
As the lightning strike electrical energy is conducted between structural joints the energy passes through the metallic fasteners. The protruding fastener head of certain fasteners have experienced arcing between the edge of the fastener head and structure in contact with the head. The arcing may be present between fastener head and the structure, or the fastener head and sleeve, if a sleeved fastener is being used.
Existing fastening systems typically require installation of the fastener with the addition of the time and labor intensive processes of installation of premolded caps over fastener heads or other procedures such as removing dielectric coatings from exposed surfaces of the fastener heads.
It is therefore desirable to provide a fastening system suitable for composite structural applications which overcomes both HPE and arcing while overcoming the drawbacks of the prior art fastener installation procedures.